The casting of molten material, e.g., a metal or a semiconductor material like silicon, among others, into a ribbon shape has been shown to have feasibility, and in the case of semiconductor material to be particularly useful for forming photovoltaic solar cell starting material. The above-referenced applications, the disclosure of which are hereby incorporated by reference, relate to a method and apparatus for such ribbon casting and a tundish and the take-up apparatus useful in the ribbon casting apparatus and method. Cast ribbon photovoltaic cells have been shown to have up to a 5.25% efficiency and are thus useful in fabricating photovoltaic cells while decreasing the manufacturing cost of fabricating the silicon or other semiconductor material into a ribbon form useful for such photovoltaic cells.
The manufacture of the photovoltaic cells includes the further step of joining the ribbon of semiconductor material to a metal backing. It is desirable for purposes of reducing the cost of manufacture of such photovoltaic cells to reduce the number of steps in the fabricating process. It is also desirable to improve the contact and back surface field effect (BSF) of the union between the metal backing and the ribbon of semiconductor material. Casting of a liquid material onto a cold solid surface normally results in fine grain structure from the rapidly frozen material. It is desirable to achieve large grain material for higher cell efficiency, and therefore it is desirable to solidify the molten material onto a liquid surface which will have fewer grain nucleation sites. Further, the ribbon of semiconductor material, due to its crystalline structure, is both fragile and brittle. It is therefore desirable to form the union of the metal backing and the semiconductor ribbon simultaneously in order to both strengthen the resulting ribbon of combined semiconductor material and metal backing and to avoid processing steps in which the semiconductor ribbon alone, without the metal backing, must be further handled and processed or transported to another location for the further processing. The method and apparatus of the present invention is also useful in fabricating cast ribbon of bimetallic or trimetallic layers. The molten material, in this instance, is also a metal and the metal foil will melt when contacted by the molten material and, depending on the type of molten material and foil and the temperature of the molten material, will form a bimetallic ribbon of cast material on the foil backing or a trimetallic ribbon of cast material composed of a backing of the foil, a layer of an alloy of the foil and cast material and a layer of crystallized cast material.
In the past it has been known to fabricate a thin ribbon of semiconductor material, e.g., silicon, with a metal backing for photovoltaic solar cell applications by slicing a thin ribbon of semiconductor material from a solid stock of semiconductor material. Thereafter one surface of the ribbon is etched, e.g., with an acid or other chemical etchant, to remove any oxide of the semiconductor material from that surface and a small amount of the semiconductor material at the surface. A metal foil backing is then applied, as is known in the art, by any of a number of methods, e.g., vacuum evaporation or printing of a metal paste on the surface, which metal paste is heat sintered onto the surface. Such prior art methods are not susceptible to continuous fabrication of a continuous ribbon of the semiconductor material with the metal backing and necessitate handling of the ribbon between the slicing step and the addition of the metal backing.
The present invention relates to the method and apparatus for fabricating a cast ribbon which reduces or substantially eliminates the problems noted above. The invention relates to the casting of a ribbon of molten material, e.g., a metal or a semiconductor material like silicon, among others, into a ribbon of crystalline or coarse-grain polycrystalline cast material by extruding the molten material onto a cooled surface, as of a rotating drum, as is more fully explained in the above-referenced patent applications. In the method of the present invention, the surface of the rotating drum carries on it a thin foil of metal, e.g., aluminum, onto which the ribbon of molten material is extruded. The molten semiconductor material is of a temperature sufficiently greater than the melting temperature of the foil of metal such that at least some portion of the foil of metal, depending upon, inter alia, its melting temperature, the rate of cooling from the drum, and the thickness of the foil, will almost instantaneously melt when contacted with the molten material. As the rotating cooled surface of the rotating drum moves away from the region of extrusion of the molten ribbon, the molten ribbon will commence crystallization on the liquified metal foil surface and the portion of the metal foil which melted will solidify. Depending upon the type of molten casting material and foil and the temperature of the molten casting material, an alloy of the cast material and foil material may also form between the crystallized cast material and the solidified foil material.
Examples of the more important features of the present invention have thus been summarized rather broadly in order that a detailed description thereof that follows may be better understood and in order that the contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereafter and which will also form the subject of the appended claims. These other features and advantages of the present invention will become more apparent with reference to the following detailed description of a preferred embodiment thereof in connection with the accompanying drawing, in which: